1. Field of the Invention
The present invention relates to fuel injectors and nozzles, and more particularly, to pure airblast fuel injectors and nozzles for gas turbine engines.
2. Description of Related Art
Pure airblast nozzles are in wide use among engine manufacturers, particularly in aircraft engines. Pure airblast nozzles create favorable air/fuel mixtures and have spray characteristics that produce combustion qualities desirable for low emissions and high engine efficiencies. A typical pure airblast injector has one fuel circuit. Fuel can be directed from an injector inlet fitting to a fuel swirler through a fuel tube. At the fuel swirler, the fuel can be split into a multitude of discrete paths, all of which discharge into the combustor. These discrete paths are all fluidly connected and are thus all part of a single fuel circuit.
One example of a pure airblast fuel injector is described in U.S. Pat. No. 6,622,488 to Mansour, et al., which shows a fuel injector having a pure airblast nozzle connected to a housing stem. The fuel nozzle includes a fuel swirler that has a plenum for receiving fuel from a conduit in the housing stem. A plurality of fuel passages conduct fuel from the plenum to discharge orifices. The downstream ends of the passages are angled so as to impart swirl on fuel exiting therethrough. A prefilmer surrounds the fuel swirler. Fuel exiting the swirler is directed inwardly by the prefilmer. An inner air passage extends through the center of the fuel swirler and an outer air passage is defined outboard of the prefilmer. The inner and outer air passages include air swirlers for imparting swirl to compressor discharge air flowing therethrough. As fuel exits the swirler/prefilmer, it is sheared between the swirling air flows issuing from the inner and outer air passages to atomize the fuel for combustion.
While pure airblast nozzles can provide for clean fuel combustion when the engine compressor is spooled up, difficulties can arise during engine startup. Pure airblast nozzles depend on fast moving air to break up the slower moving fuel spray into fine droplets. As described above, the airblast typically comes from compressor discharge air routed through the nozzle. However, during engine startup the compressor is not fully spooled up and thus the air pressures provided to the nozzle during engine startup are not always high enough to provide the necessary atomizing air blast. Therefore, the amount of fuel atomized can be insufficient to initiate or sustain ignition. Thus it can be quite difficult to start an engine using only traditional pure air blast nozzles.
Another problem during startup for traditional airblast nozzles is that when the startup air pressure is too low to filly atomize the fuel flowing from the nozzle, significant amounts of fuel can issue from the nozzle without being atomized. Liquid fuel drooling from the nozzle constitutes waste of fuel and can cause poor emissions as well as complications that can arise from fuel pooling in undesirable locations of the engine. Pooled fuel can ignite explosively and emit a plume of white smoke out of the exhaust.
Some solutions to these problems have been suggested, such as including auxiliary start nozzles, for example liquid-pressure atomizing nozzles, dedicated for use during start up. Other solutions include adding auxiliary air pumps or compressors to generate atomizing air blasts through pure airblast nozzles during engine start up. It is also known to use hybrid nozzles, which include air blast fuel nozzles for fall power operation in addition to liquid-pressure atomizing nozzles for use during startup. However, while these solutions can be used to facilitate engine start up, they can also add significantly to the cost and weight of the engine.
Piloted airblast nozzles are sometimes used to achieve the needed starting characteristics while trying to match pure airblast nozzle performance. However, piloted airblast nozzles tend to lack the superior thermal management inherent in pure airblast nozzles. Piloted airblast nozzles also fail to achieve identical spray characteristics with pure airblast nozzles because the pressure atomizing circuit mixes with the airblast spray.
Such conventional methods and systems generally have been considered satisfactory for their intended purpose. However, there still remains a continued need in the art for a nozzle or fuel injector that allows for improved startup performance and provides the benefits of pure airblast atomization. There also remains a need in the art for such a nozzle or injector that is easy to make and use. The present invention provides a solution for these problems.